Jet fuel process

ABSTRACT

A PROCESS FOR PRODUCING AN IMPROVED JET A OR JET A-1 AVIATION TURBINE FUEL HAVING INCREASED API GRAVITY AND SMOKE POINT AND REDUCED AROMATICS CONTENT BY SEPARATING A FULL BOILING RANGE JET FUEL INTO (A) A LIGHT FRACTION BOILING FROM ABOUT 290*F. TO ABOUT 400*F., (B) A MIDDLE FRACTION BOILING FROM ABOUT 400*F. TO ABOUT 480*F., AND (C) A HEAVY FRACTION BOILING FROM ABOUT 480*F. UP TO ABOUT 550*F.; THEN COMBINING FRACTIONS (A) AND (C) TO OBTAIN AN IMPROVED FULL BOILING RANGE JET FUEL. OPTIONALLY FRACTION (B) MAY BE TREATED FRO REMOVAL OR CONVERSION OF NAPHTHENO-AROMATIC COMPOUNDS AND THEN COMBINED WITH FRACTIONS (A) AND (C) TO INCREASE BOTH VOLUME AND QUANTITY OF THE JET FUEL.

United States Patent O U.S. Cl. 208-93 5 Claims ABSTRACT OF THE DISCLOSURE A process for producing an improved Jet A or let A-l aviation turbine fuel having increased A-PI gravity and smoke point and reduced aromatics content by separating a full boiling range jet fuel into (a) a light fraction boiling from about 290 F. to about 400 F., (b) a middle fraction boiling from about 400 F. to about 480 F., and (c) a heavy fraction boiling from about 480 F. up to about 550 F.; then combining fractions (a) and (c) to obtain an improved full boiling range jet fuel. Optionally fraction (b) may be treated for removal or conversion of naphtheno-aromatic compounds and then combined with fractions (a) and (c) to increase both volume and quantity of the jet fuel. J

BACKGROUND OF THE INVENTION Field This invention relates to a process for improving aviation turbine fuels and more particularly to a process for increasing API gravity and smoke point and decreasing aromatics content without hydrogenation.

Prior Art To produce a specification jet fuel it is necessary to carefully tailor the various components of the final blend to simultaneously meet all requirements. Typical jet fuel fractions boil in the range from about 290 F. to 550 F. Within this boiling range are generally found types of hydrocarbon compounds which make it difiicult to simultaneously meet such specifications as API gravity, smoke point and aromatics content without additional treatment of the fraction.

Various means have been proposed to accomplish the tailoring operation. For example, in US. Pat. 3,172,833 high luminometer numbers, high smoke points and loW freeze points are obtained by hydrocracking a hydrocarbon distillate feed boiling in the range from about 500 F. to about 1050 F., separating a jet fuel fraction boiling in the range of 320 F. to 550 F. from the hydrocracked product, and hydrogenating from about 5 to 100% v. of the jet fuel fraction.

The proper tailoring for the production of a high performance decalinic jet fuel for supersonic aircraft having high temperature stability and maximum energy content is accomplished in US. Pat. 3,126,330 by first hydroforming a naphtha having an end point between 380 F. to 440 F. to produce a hydroformed naphtha and a higher boiling aromatic oil. The aromatic oil is then distilled to obtain a fraction boiling from about 450 F. to 570 F. and finally the highly aromatic is hydrogenated to substantially complete saturation. For this supersonic jet fuel an extremely low aromatics content, not substantially higher than about 0.5% v., is required. Obviously, very low quantities of this type of jet fuel can be produced by such a process.

A high density supersonic jet fuel can also be prepared by blending components having the desired properties. For example, in US. Patent Re. 26,116 a blend of a mixture of Decalin and alkyl Decalin with about to 25% hydrogenated propylene tetramer provides a fuel having satisfactory fuel density, thermal stability and viscosity for supersonic aircraft. The Decalins used are preferably prepared from hydrogenated polycyclic hydrocarbons which having been extracted from a catalytically cracked petroleum fraction boiling from about 400 to 520 F. This patent discloses that the 400 F. to 520 F. boiling range fraction is critical for the polycyclic aromatic fraction in that higher boiling fractions do not have the thermal stablity required and they possess widely varying viscosity characteristics.

A more recent proposal for producing a high density supersonic jet fuel having a low freeze point and a high heat of combustion accomplishes this by properly proportioning the monocyclic naphthenes to the polycyclic naphthenes and by using a larger amount of parafiins. US. Pat. 3,367,860 discloses that a jet fuel may be produced by blending about two parts of a straight run stove oil from a naphthenic petroleum stock of a specified composition with about one part of a stove oil from an intermediate petroleum stock of another specified composition. The resulting blend is then catalytically hydrogenated to about 95% saturation and distilled to separate a 370 F. to 5 20 F. heart-cut therefrom. This heart-cut fraction is then further hydrogenated to about 98% saturation to produce a high density fuel having a mononaphthenic to polynaphthenic ratio greater than 1:1 and preferably about 2:1.

The content of polycyclic naphtheno-aromatics and substituted polycyclic naptheno-arornaties in any given jet fuel fraction will depend upon the crude oil source as Well as the prior processing steps from which the jet fuel has been derived. Jet fuels containing an excess of these compounds generally exhibit API gravities and smoke points which are too low and an aromatics content which is too high to meet ASTM specifications. One way to improve such jet fuels is to hydrogenate all or a portion of the jet fuel to convert the polycyclic naphtheno-aromatic compounds to polycyclic naphthenes. For example Tetralin has a boiling point of 403 F. and a specific gravity of 0.973. By hydrogenating this compound to Decalin the boiling point is reduced to 365-380 F. and the specific gravity to 0872 0.895. Simultaneously, the smoke point has been increased and the aromatics content reduced. However, when a jet fuel is hydrogenated to obtain this improvement, it may require refractionation to remove light components from side reactions such as cracking.

The substituted polycyclic naphtheno-aromatic compounds which are deleterious to jet fuel are found in the heavy portion of jet fuel boiling from about 400 F. to about 500 F. Attempts to improve jet fuel by removing the high boiling bottoms portion and back-blending a portion thereof have resulted in improved gravity, smoke point and aromatics content, but have failed to meet distillation specifications.

In attempting to meet the ASTM D-1655 specifications for Jet Fuel A and Jet Fuel A1, it is frequently necessary to reduce fuel density, i.e., increase API gravity, to reduce aromatics content and to increase smoke point. This can be accomplished of course by hydrogenating the polycyclic aromatics in the jet fuel, such as Tetralin and alkyl Tetralin to Decalin and alkyl Decalin, in all or a fraction of the jet fuel. This procedure may require additional fractionation facilities to remove light ends from the hydrogenated jet fuel. A separation and blending process has now been discovered which provides considerable operating flexibility for improving jet fuel properties without hydrogenating the polycyclic aromatics While still meeting distillation specification.

3 THE INVENTION The jet fuels to which the present invention is applicable are specified as ASTM Jet A and Jet A-l. Complete specifications for these jet fuels may be found in the ASTM Specifications for Aviation Turbine Fuels, set forth as ASTM D-1655-70. Pertinent properties from this specification are included in the examples for comparative purposes. This invention is applicable to jet fuels bil-* ing in the range from about 290 F. up to about 550 F. which do not meet the Jet A or Jet A-1 specifications for API gravity, smoke point or aromatics content by reason of an excess concentration of polycyclic naphtheno-aromatic compounds, e.g., indane or Tetralin.

The process of the invention consists essentially of separating a jet fuel fraction having an ASTM boiling range from about 290 F. to about 550 F. into (a) a light fraction boiling from about 290 F. to about 400 F., (b) a middle fraction boiling from about 400 F. to about 480 F., and (c) a heavy fraction boiling from about 480 F. up to maximum end point of 550 F., and then combining fractions (a) and (c) to form a full boiling range jet fuel. An improved jet fuel is obtained which also meets distillation specifications. This beneficial result is obtained because removal of a relatively flat middle portion on the distillation curve from a jet fuel has only a minor eifect on the overall distillation specifications for ASTM Jet A and Jet Al.

The improvement in jet fuel properties effected by the process of the invention results in some decrease in jet fuel volume. The higher the concentration of naphthenoaromatic compounds the greater will be the reduction in volume required to meet jet fuel specifications. The final boiling point and volume of the middle fraction to be removed can be readily determined in the laboratory by separating the jet fuel to be improved into several fractions and comparing the properties of appropriate blends with ASTM D-l655 jet fuel specifications.

The middle fraction of jet fuel from the separation operation (b) can be disposed of in any of several ways. For example, it can be used as a blending component for furnace oil or as a cutter stock for asphalt. Preferably, however, this fraction is treated to remove naphtheno-aromatic compounds or to convert them to polycyclic naphthenes and to combine the treated fraction with the light and heavy fractions to produce an increased volume of even higher quality jet fuel. The naphtheno-aromatics may be removed, for example, by extraction with a suitable solvent.

Although any part or all of the jet fuel fraction can be hydrogenated to obtain improved properties, hydrogenation of the middle fraction (b) referred to above to convert substantially all of the naphtheno-aromatics to polycyclic naphthenes makes the most efficient use of hydrogen. The general practice heretofore has been to severely hydrogenate only a part, e.g., v., of a full boiling range jet fuel and then to blend the hydrogenated portion back into the remaining jet fuel. This practice is ineificient since hydrogenation of the lower boiling portion of jet fuel results in very little improvement of desired properties. The mono-aromatics which are contained in the lower boiling portion of jet fuel, have very little adverse effect on the critical properties of jet fuel such as smoke point so that hydrogenating them to naphthenes uses up valuable hydrogen without appreciable benefit.

This invention shows that a significant improvement in jet fuel quality can be obtained by removing a middle fraction from a full boiling range jet fuel. An even greater improvement can be realized by hydrogenating this middle fraction and blending it with the light and heavy boiling fractions. In both instances, improvements in quality are obtained with only minor effects on the overall boiling point characteristics of the jet fuel. Thus undesirable variations in boiling point characteristics are avoided while higher yields of high quality jet fuel are realized.

It is apparent that increased volume of jet fuel can also be obtained by substituting another hydrocarbon fraction boiling in the range of 400 F. to 480 F. which is low in harmful polycyclic naphtheno-aromatics compound, if such a fraction is available.

This invention is especially applicable to jet fuels which are obtained from a hydrocracking process. Feedstocks to such processes include both straight run and catalytically cracked distillates boiling from about 400 F. to about 1050 F. Frequently, relatively high concentrations of naphtheno-aromatic compounds are found in these jet fuels. However, it is not essential to the invention that the jet fuel fraction be obtained from hydrocracking processes. If the jet fuel fraction fails to meet Jet A-1 gravity, smoke point or aromatics content specifications because of polycyclic naphtheno-aromatic compounds, then the process of the invention is applicable.

Considerable operating flexibility is provided by the process of the invention. By separating a jet fuel into appropriate fractions and removing the middle fraction, an improvement of at least one unit higher API gravity, at least one unit higher smoke point and at least 2% wt. lower aromatics content can be realized without altering the overall distillation characteristics of the jet fuel. If additional volume of even higher quality jet fuel is required the middle fraction may be removed, treated to remove or convert the harmful polycyclic naphtheno-aromatics compounds and then recombined with the light and heavy fractions.

The following examples illustrate the invention and its advantages but are not to be construed as limiting the invention to the embodiments disclosed.

EXAMPLE I This example demonstrates that to meet jet fuel specifications by removing a heavy (high boiling) portion from a full range jet fuel fraction and back-blending part of the heavy portion with the light portion results in an excessive yield loss.

A West Coast straight run heavy gas oil boiling above 596 F., having an API gravity of 23.5 and containing about 1400 p.p.m.w. total nitrogen, was hydrocracked in a single stage catalytic process. The feedstock was hydrocracked to 67% conversion to products boiling below 520 F. at a temperature of 720 F., a pressure of 1400 p.s.i..g., a liquid hourly space velocity of 1.4, and a hydrogen to oil molar ratio of 12:1. All hydrocracked product boiling above the jet fuel range (about 520 F.) was recycled to the process feed stream.

A full range jet fuel boiling from about 300 F. to 520 F. was separated from the hydrocracked product on a 20-plate Oldershaw distillation column. This jet fuel was further separated into a light fraction boiling from 297 F. to 392 F. and a heavy fraction boiling from 392 F. to 520 F. One-third of the heavy fraction was combined with the light fraction to produce a jet fuel fraction having a boiling range from about 324 F. to 504 F. The combined fractions were 60% Wt. of the full range jet fuel. Selected properties of the full range jet fuel fraction and of the combined jet fuel fraction are compared with ASTM D 1655-70 Jet A and Jet A-l Aviation Turbine Fuel specifications in Table l.

Nora-Figures underlined fail specification.

These data show that by removing two-thirds of the heavy fraction from the jet fuel a considerable improvement in API gravity, smoke point and aromatics content has resulted. However, this improvement was achieved at the expense of a 40% wt. reduction in the total jet fuel product.

EXAMPLE II To illustrate the process of the invention the feedstock of Example I was hydrocracked in a similar single-stage catalytic process under the same reaction conditions. A full-range jet fuel boiling from 297 F. to 520 F. was again separated from the hydrocracked product on the 20-plate Oldershaw column. This jet fuel was then further separated by distillation into three fractions: (a) a light fraction boiling from 297 F. to 428 F.; (b) a middle fraction boiling from 428 F. to 480 F.; and (c) a heavy fraction boiling from about 480 F. to 520 F. The light and heavy fractions were then combined to obtain an improved jet fuel, having an ASTM boiling range from about 331 F. to 532 F. The combined fractions were 75% Wt. of the full range jet fuel. Selected properties of the full range jet fuel and of the combined jet fuel fractions (a) and (c) are shown in Table 2.

TABLE 2 Combined ASTM JetAand light J'et A 1 Full fraction range plus jet heavy fraction specifications Minl- Maximum mum N Urn-Figures underlined tail specification.

These data show that by removing a middle fraction (b) boiling from about 428 F. to 480 F. from the jet fuel, at least a one-unit improvement was realized in API gravity and smoke point, and at least a two percent weight reduction in aromatics content. In addition, increased quantities of jet fuel were obtained over the blending method of Example I (75 wt. vs. 60% wt.).

A further improvement in jet fuel quality and an increase in jet fuel volume can be achieved by treating the middle jet fuel fraction (b) to remove or convert compounds deleterious to jet fuel (e.g., alkyl Tetralins, etc.) and then combining the middle fraction with the light and heavy fractions to obtain a full range jet fuel.

What is claimed is: v

1. A process for producing an improved Jet A or Jet A-1 aviation turbine fuel from a hydrocarbon distillate having a boiling range from about 290 F. to about 550 F. and containing a relatively high concentration of polycyclic naphtheno-aromatic compounds, said improvement consisting of at least a one unit' increase in API gravity and smoke point and at least a 2% wt. reduction in aromatics content, which consists essentially of separating said hydrocarbon distillate into (a) a light fraction boiling from about 290 F. to about 400 F., (b) a middle fraction boiling from about 400 F. to about 480 F., and (c) a heavy fraction boiling from about 480 F. to 550 F.; and then combining fractions (a) and (c) to obtain said improvement.

:2. The process of claim 1 wherein the hydrocarbon distillate is obtained by hydrocracking a hydrocarbon feed boiling in the range from about 400. F. to about 1050 F. and separating said distillate from the hydrocracked product.

3. The process of claim 1 wherein the middle fraction (b) is treated to reduce the quantity of naphtheno-aromatic compounds contained therein and is then recombined with the light and heavy fractions (a) and (c) to produce an increase volume of improved jet fuel.

4. The process of claim 3 wherein the quantity of naphtheno-aromatics compounds is reduced by at least 50% wt. by contacting the middle fraction (b) with a hydrogenation catalyst in the presence of hydrogen under suit-able hydrogenation conditions.

5. The process of claim 1 wherein a hydrocarbon frac* tion having a substantially lower naphtheno-aromatic compound content than said middle fraction (b) and boiling from about 400 F. to 480 F. having been obtained from a source other than by separation from said hydrocarbon distillate is combined with the light and heavy fractions (a) and (c) to produce an increased volume of improved quality full boiling range jet fuel.

References Cited UNITED STATES PATENTS 2,729,596 1/ 1956 Mills 20815 2,749,225 6/ 1956 Barnum et al 20815 2,910,426 10/ 1959 Gluesenkamp et al. 20815 3,216,929 11/1965 Favre 208-15 3,236,764 2/ 1966 Den Herder et al. 208--15 HERBERT LEVINE, Primary Examiner US. Cl. X.R. 208-15, 93 

